When you clay the pistons to get an imprint of the valve, measure the imprint with a micrometer and you should have a minimum of .050" thickness. Anything thinner could translate to eventual valve/piston clash if your valves should happen to float.
At higher RPM your valves can actually stay open at max lift for a longer period of time as the lifter may actually come off the lobe when it hits the return side of the lobe. when this happens, thats when you could wind up with a valve coming in contact with a piston. Just be sure you have the correct tension springs for your cam and dont try over revving it; turnging it higher than the grinders recommended rpm range.